Method and system for diagnosing state of fuel cell

ABSTRACT

A method and system for diagnosing a state of fuel cell are provided. The system includes a signal measurement unit that has a high pass filter with a predetermined cut-off frequency and a voltage measurement circuit, and that measures a first AC voltage to measure the fuel cell state diagnosis signal and a noise measurement unit including a band pass filter that has a predetermined pass band and a voltage measurement circuit, and that measures a second AC voltage to measure the fuel cell state diagnosis noise. A controller calculates a signal to noise ratio (SNR) of fuel cell state diagnosis data based on the first and second AC voltages, determines the corresponding fuel cell state diagnosis data to be reliable when the SNR value is greater than a predetermined reference value, and applies the fuel cell state diagnosis data to a control of a fuel cell vehicle.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean PatentApplication No. 10-2015-0178293 filed in the Korean IntellectualProperty Office on Dec. 14, 2015, the entire contents of which areincorporated herein by reference.

BACKGROUND

(a) Field of the Invention

The present invention relates to a method and system for diagnosing astate of a fuel cell, and more particularly, to a method and system fordiagnosing a state of a fuel cell to improve the reliability ofdiagnosis by calculating a signal to noise ratio.

(b) Description of the Related Art

Generally, a fuel cell includes an electrode that provokes anelectrochemical reaction, a polymer electrolyte membrane that transfershydrogen ions generated by the reaction and a separator that supportsthe electrode and the polymer electrolyte membrane. The polymerelectrolyte fuel cell (hereinafter, referred to a fuel cell) has meritsin that the fuel cell is high in efficiency compared with other shapesof fuel cells, great in the current density and the output density, theignition time is short, and at the same time, it is not corroded norrequired for the regulation of electrolyte since the polymer electrolytefuel cell uses solid electrolyte. In addition, since the fuel cell is anenvironmentally friendly power source that produces no exhaust exceptonly pure water, currently research is being conducted in automobileindustries.

Commonly, the fuel cell life span and performance are influenced by thedriving condition of the fuel cell. Accordingly, in the relatedindustry, the diagnosis of fuel cell state is performed to obtain theinformation for maintaining the driving condition of the fuel cell inthe optimal condition. Generally, the method for diagnosing the state offuel cell includes a method of measuring the current-voltage curve and amethod of measuring the impedance of the fuel cell. The measurement offuel cell impedance is commonly measured by flowing alternating current(AC) current through the fuel cell or calculating the impedance of thefuel cell by outputting AC current from the fuel cell and measuring ACvoltage of the fuel cell. In particular, the following relation isestablished generally.

$\begin{matrix}{\frac{V_{ac}(f)}{I_{ac}(f)} = {Z(f)}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

wherein, V_(ac)(f) represents the AC voltage of the fuel cell atfrequency f, I_(ac)(f) represents the AC current of the fuel cell atfrequency f, and Z(f) represents the impedance of the fuel cell atfrequency f. Meanwhile, it is common that the voltage of the fuel cellchanges significantly by acceleration and deceleration of the fuel cellvehicle. The voltage spectrum of the fuel cell spreads to a wide range.In addition, owing to the operation frequencies of the motor, theinverter and the converter mounted on the fuel cell vehicle, the voltageof the fuel cell has the spectrum of very diverse and irregular.

Accordingly, when the spectrum of the fuel cell voltage and thefrequency range for diagnosing the fuel cell state are overlapped, thereliability of diagnosis is degraded by increasing the measurement errorof the diagnosis of the fuel cell. In the fuel cell vehicle environment,an effort for improving the accuracy of the fuel cell impedancemeasurement has been conducted since the performance, the durability andthe usability of the fuel cell is related to the fuel cell state.However, more reliable measurement technique of the fuel cell impedanceis needed since the conventional measurement technique of impedance hasa problem of degrading reliability due to the big measurement error ofdiagnosing the fuel cell state due to the problems described above.

The above information disclosed in this section is merely forenhancement of understanding of the background of the invention andtherefore it may contain information that does not form the prior artthat is already known in this country to a person of ordinary skill inthe art.

SUMMARY

The exemplary embodiments of the present invention is to provide amethod and system for diagnosing a state of fuel cell, which may improvethe reliability of diagnosing the state of fuel cell by calculating asignal to noise ratio (SNR) by defining the frequency band neighboringthe signal except the diagnosis signal frequency of the fuel cell stateas a noise region and using the calculated SNR for vehicle control whenthe SNR is greater than a reference value to improve the reliability ofdiagnosing the state of fuel cell.

A system for diagnosing a state of fuel cell according to an exemplaryembodiment of the present invention may include a signal measurementunit having a high pass filter with a predetermined cut-off frequencyand a voltage measurement circuit, and configured to measure a first ACvoltage for measuring the fuel cell state diagnosis signal; a noisemeasurement unit having a band pass filter with a predetermined passband and a voltage measurement circuit, and configured to measure asecond AC voltage for measuring the fuel cell state diagnosis noise; anda controller configured to calculate a signal to noise ratio (SNR) offuel cell state diagnosis data based on the first AC voltage and thesecond AC voltage, determine the corresponding fuel cell state diagnosisdata to be reliable when the SNR value is greater than a predeterminedreference value, and apply the fuel cell state diagnosis data to acontrol of a fuel cell vehicle.

When the SNR value is less than the reference value, the controller maydiscard the fuel cell state diagnosis data. The noise measurement unitmay define the band neighboring the signal frequency except the fuelcell state diagnosis signal frequency to be a noise region. Thecontroller may further be configured to calculate the SNR by dividingthe signal region of the first AC voltage by the noise region of thesecond AC voltage. The signal measurement unit may have the cut-offfrequency of about 980 Hz that is close to the state diagnosis frequency1 kHz, thereby removing a hum noise caused by the noise near to the fuelcell state diagnosis frequency.

A system for diagnosing a state of fuel cell according to an exemplaryembodiment of the present invention may include a signal measurementunit having a high pass filter with a predetermined cut-off frequencyand a voltage measurement circuit, and configured to measure a first ACvoltage for measuring the fuel cell state diagnosis signal; a noisemeasurement unit configured to measure a high voltage of a fuel cellthrough a voltage dividing circuit, and measure a second AC voltage formeasuring the fuel cell state diagnosis noise; and a controllerconfigured to calculate a signal to noise ratio (SNR) of fuel cell statediagnosis data based on the first AC voltage and the second AC voltage,determine the corresponding fuel cell state diagnosis data to bereliable when the SNR value is greater than a predetermined referencevalue, and apply the fuel cell state diagnosis data to a control of afuel cell vehicle.

The noise measurement unit may be configured to measure a voltagedeviation that is a difference between a maximum voltage value and aminimum voltage value among the second AC voltage values. The controllermay be configured to determine the corresponding fuel cell statediagnosis data to be reliable when the voltage deviation is less than apredetermined reference deviation while the SNR value is greater than apredetermined reference value. The noise measurement unit may further beconfigured to measure the high voltage of fuel cell by decreasing thevoltage through the voltage dividing circuit.

A system for diagnosing a state of fuel cell according to an exemplaryembodiment of the present invention may include an integratedmeasurement unit having a high pass filter with a predetermined cut-offfrequency and a voltage measurement circuit, and configured to measure afirst AC voltage for measuring the fuel cell state diagnosis signal anda second AC voltage for measuring the fuel cell state diagnosis noise;and a controller configured to calculate a signal to noise ratio (SNR)of fuel cell state diagnosis data based on the first AC voltage and thesecond AC voltage, determine the corresponding fuel cell state diagnosisdata to be reliable when the SNR value is greater than a predeterminedreference value, and apply the fuel cell state diagnosis data to acontrol of a fuel cell vehicle.

A method for diagnosing a state of fuel cell by a system for diagnosinga state of fuel cell according to an exemplary embodiment of the presentinvention may include measuring a first AC voltage for measuring thefuel cell state diagnosis signal by collecting fuel cell voltage signalsgreater than a predetermined cut-off frequency; measuring a second ACvoltage for measuring the fuel cell state diagnosis noise by collectingfuel cell voltage signals of a predetermined pass band; calculating asignal to noise ratio (SNR) of fuel cell state diagnosis data based on asignal region of the first AC voltage and a noise region of the secondAC voltage; and determining the corresponding fuel cell state diagnosisdata to be reliable when the SNR value is greater than a predeterminedreference value, and applying the fuel cell state diagnosis data to acontrol of a fuel cell vehicle.

In the calculation of the signal to noise ratio (SNR) of fuel cell statediagnosis data, the SNR may be calculated by defining the bandneighboring the signal frequency except the fuel cell state diagnosissignal frequency to be the noise region. The method may further includemeasuring a voltage deviation that is a difference between a maximumvoltage value and a minimum voltage value among the second AC voltagevalues. The method may further include comparing the voltage deviationwith a predetermined reference deviation while the SNR value is greaterthan the predetermined reference value; and determining thecorresponding fuel cell state diagnosis data to be reliable when thevoltage deviation is less than the predetermined reference deviation. Inaddition, the method may include discarding the fuel cell statediagnosis data when the SNR value is less than the reference value.

According to the exemplary embodiments of the present invention, thereliability of fuel cell state diagnosis may be improved by calculatingthe SNR value of the fuel cell state diagnosis data which are calculatedthrough the respective measurement voltages of the signal measurementunit and the noise measurement unit and applying the state diagnosisdata to the vehicle diagnosis and control when the SNR value is greaterthan a predetermined value. In addition, the noise related to accuratediagnosis may be removed by determining the state diagnosis data thatsatisfies the condition that the voltage deviation of the noise is lessthan the predetermined reference deviation to be reliable. Furthermore,by improving the reliability of fuel cell state diagnosis data throughthe fuel cell voltage measurement scheme, the control performance anddurability of the fuel cell vehicle may be improved and the customersatisfaction measurement may be increased.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description when taken in conjunction with the accompanyingdrawings, in which:

FIG. 1A-1B illustrate operation frequency due to the conventionalelectric power compartments according to the related art;

FIG. 2 is a graph illustrating the existing interference on thefrequency of diagnosing the state of fuel cell according toacceleration/deceleration of the vehicle according to the related art.

FIG. 3 illustrates the conventional measurement result of the fuel cellvoltage in high speed when driving a fuel cell vehicle according to therelated art;

FIG. 4 is a block diagram schematically illustrating a construction of afuel cell state diagnosis system according to an exemplary embodiment ofthe present invention;

FIGS. 5A-5B illustrate the SNR defined according to an exemplaryembodiment of the present invention which is compared with theconventional SNR;

FIG. 6 is a block diagram illustrating a voltage measurement device forthe fuel cell state diagnosis according to a first exemplary embodimentof the present invention;

FIG. 7 is a flowchart illustrating a fuel cell state diagnosis methodusing a voltage measurement device according to the first exemplaryembodiment of the present invention;

FIG. 8 is a block diagram illustrating a voltage measurement device forthe fuel cell state diagnosis according to a second exemplary embodimentof the present invention;

FIG. 9 is a flowchart illustrating a fuel cell state diagnosis methodusing a voltage measurement device according to the first exemplaryembodiment of the present invention;

FIG. 10 is a block diagram illustrating a voltage measurement device forthe fuel cell state diagnosis according to a third exemplary embodimentof the present invention;

FIG. 11 is a flowchart illustrating a fuel cell state diagnosis methodusing a voltage measurement device according to the third exemplaryembodiment of the present invention; and

FIGS. 12A-12B illustrate an operational effect of a voltage measurementdevice according to an exemplary embodiment of the present invention.

DESCRIPTION OF SYMBOLS

-   -   100: fuel cell state diagnosis system    -   110: fuel cell    -   120: signal measurement unit    -   130: noise measurement unit    -   140: controller    -   150: integrated measurement unit

DETAILED DESCRIPTION

It is understood that the term “vehicle” or “vehicular” or other similarterm as used herein is inclusive of motor vehicles in general such aspassenger automobiles including sports utility vehicles (SUV), buses,trucks, various commercial vehicles, watercraft including a variety ofboats and ships, aircraft, and the like, and includes hybrid vehicles,electric vehicles, plug-in hybrid electric vehicles, hydrogen-poweredvehicles and other alternative fuel vehicles (e.g. fuels derived fromresources other than petroleum). As referred to herein, a hybrid vehicleis a vehicle that has two or more sources of power, for example bothgasoline-powered and electric-powered vehicles.

Although exemplary embodiment is described as using a plurality of unitsto perform the exemplary process, it is understood that the exemplaryprocesses may also be performed by one or plurality of modules.Additionally, it is understood that the term controller refers to ahardware device that includes a memory and a processor. The memory isconfigured to store the modules and the processor is specificallyconfigured to execute said modules to perform one or more processeswhich are described further below.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof. As used herein, the term “and/or”includes any and all combinations of one or more of the associatedlisted items.

Unless specifically stated or obvious from context, as used herein, theterm “about” is understood as within a range of normal tolerance in theart, for example within 2 standard deviations of the mean. “About” canbe understood as within 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%,0.1%, 0.05%, or 0.01% of the stated value. Unless otherwise clear fromthe context, all numerical values provided herein are modified by theterm “about.”

Hereinafter, by reference to the accompanying drawings, the exemplaryembodiments of the present invention will be described in detail suchthat an ordinary skilled person in this art may easily implement.However, the present invention may be implemented in various forms, andthe scope of the present invention is not limited to the embodimentsdescribed herein. And, in the drawings, to clearly describe the presentinvention, parts which are not in relation to the description areomitted, and the same reference numerals are designated for identical orsimilar elements throughout the specification.

When a part “includes” a certain element, this means that the part maynot exclude other elements but further include them throughout thespecification, unless any specific opposite description is presented. Inaddition, the term such as “part”, “unit”, “module”, etc. described inthe specification means a unit that processes at least one function oroperation, and may be implemented by hardware, software or thecombination thereof. Throughout the specification, the parts shown byidentical reference numerals represent the identical elements.

Now, a method and apparatus for measuring voltage for diagnosing thefuel cell state according to an exemplary embodiment of the presentinvention will be described in detail by reference to the accompanyingdrawings. Before describing the present invention, the cause ofdegrading the accuracy of measuring the impedance of fuel cell whendriving a fuel cell vehicle will be described. The degrading of theaccuracy of measuring the impedance of fuel cell when driving a fuelcell vehicle is rooted in the noise due to electric power compartmentsand the noise due to the stack voltage fluctuation according toacceleration/deceleration of the vehicle.

First, the noise due to electric power compartments may be caused by thecompartments including a high voltage direct current-direct current(DC-DC) converter, a low voltage DC-DC converter and an inverter. Suchelectric power compartments have irregular operation frequencies, andmore irregular frequency characteristics are shown since the operationsof the electric power compartments are coupled with each other.Accordingly, it may be difficult to remove the noise due to the electricpower compartments using the frequency for diagnosing the state of fuelcell by avoiding frequencies of a specific range.

For example, FIGS. 1A-1B illustrate an example of operation frequencydue to the conventional electric power compartments according to therelated art. Referring to FIG. 1A, the noise due to the electric powercompartments has the shape including specific peaks, and does not invadethe frequency of diagnosing the state of fuel cell. On the contrary, inFIG. 1B, since the noise due to the electric power compartments has theband shape including the frequency of diagnosing the state of fuel cell,it may be difficult to remove the noise. Further, the noise due to thestack voltage fluctuation according to acceleration/deceleration of thevehicle has wide and strong spectrum on the characteristics, and thespectrum interferes in the frequency of diagnosing the state of fuelcell and degrades the reliability of diagnosing the state of fuel cell.

For example, FIG. 2 is a graph illustrating the existing interference onthe frequency of diagnosing the state of fuel cell according toacceleration/deceleration of the vehicle in the related art. Referringto FIG. 2, 0.01 V amplitude of the frequency of diagnosing the state offuel cell of 22 Hz becomes 0.03894 V due to the constructiveinterference or 0.01894 V owing to the destructive interference sincethe noise due to the fuel cell voltage fluctuation is added.Accordingly, the interference due to the noise degrades the reliabilityof diagnosing the state of fuel cell.

Meanwhile, FIG. 3 illustrates the conventional measurement result of thefuel cell voltage when driving a fuel cell vehicle according to therelated art. Referring to FIG. 3, the problems of the noise due to theelectric power compartments and abrupt falling of the fuel cell voltageare observed in a real fuel cell vehicle. FIG. 4 is a block diagramschematically illustrating a construction of a fuel cell state diagnosissystem according to an exemplary embodiment of the present invention.Referring to FIG. 4, the fuel cell state diagnosis system 100 accordingto an exemplary embodiment of the present invention may include a signalmeasurement unit 120, a noise measurement unit 130 and a controller 140.The controller 140 may be configured to operate the signal measurementunit 120 and the noise measurement unit 130.

In particular, the fuel cell 110 may be made up of a polymer electrolytefuel cell. The signal measurement unit 120 may be configured to measurethe voltage (hereinafter, referred to a first AC voltage) to determinethe state diagnosis signal of a fuel cell stack 110. The noisemeasurement unit 130 may be configured to measure the voltage(hereinafter, referred to a second AC voltage) to detect noise. Thecontroller 140 may be configured to execute overall operations ofrespective elements for fuel cell state diagnosis according to anexemplary embodiment of the present invention. In particular, thecontroller 140 may define the signal to noise ratio (SNR) of the statediagnosis data of fuel cell based on the AC voltage signals measured inthe signal measurement unit 120 and the noise measurement 130, and maybe configured to determine the reliability of fuel cell state diagnosisresult based on the SNR value.

For example, the controller 140 may be configured to determine the statediagnosis data of fuel cell to be reliable when the SNR value calculatedusing the signal measurement unit 120 and the noise measurement unit 130is greater than a predetermined reference value, and apply the statediagnosis data to the control (driving) of the fuel cell vehicle.Additionally, the controller 140 may be configured to discard the statediagnosis data when the SNR value is less the reference value. Thecontroller 140 may further be configured to determine the statediagnosis data of fuel cell to be reliable when the difference V_(fc)between the maximum value and the minimum value among the fuel cellvoltages measured using the noise measurement unit 130 is less than apredetermined reference deviation, and apply the state diagnosis data tothe control (driving) of the fuel cell vehicle. The controller 140 maybe configured to discard the state diagnosis data when the difference isless the reference deviation.

Meanwhile, FIGS. 5A-5B illustrate the SNR defined according to anexemplary embodiment of the present invention which is compared with theconventional SNR. Referring to FIGS. 5A-5B, the SNR according to anexemplary embodiment of the present invention is defined as Equation 2below.SNR=Amplitude(f _(signal))/Amplitude(f _(signal))Signal=Amplitude(f _(signal)), f _(signal): Frequency of fuel cell statediagnosis signalNoise=Amplitude(f _(signal)), f _(signal) : A˜f _(signal)+Region of B(excluding f _(signal))  Equation 2

In particular, the noise region of the noise measurement unit 130 isdefined as the band neighboring the signal frequency except the statediagnosis signal frequency. Such a scheme of the SNR definitionaccording to an exemplary embodiment of the present invention isdistinguished from the SNR definition scheme in which the noise isdefined in the same frequency of the signal shown in Equation 3 below.Normal SNR=Amplitude(f _(signal))/Amplitude(f _(noise))Signal=Amplitude(f _(signal)), Measured in signal transmission unitNoise=Amplitude(f _(noise)), Measured in signal reception unit  Equation3

Meanwhile, the fuel cell state diagnosis system 100 may further includea signal generation unit operated by the controller to apply AC currentfor diagnosis to the fuel cell 10. Accordingly, the controller 140 maybe configured to diagnose the state of fuel cell and/or breakdown withimproved reliability by determining whether to use the state diagnosisdata based on the SNR value calculated using the respective AC voltagesmeasured in the signal measurement unit 120 and the noise measurementunit 130 in the state that the current of the fuel cell stack 110 andthe diagnosing AC current of the signal generation unit are overlappedand flow to a load (not shown).

Furthermore, the structure of the signal measurement unit 120 and thenoise measurement unit 130 included in the voltage measurement devicefor improving the reliability of fuel cell state diagnosis of the fuelcell state diagnosis system 100 according to an exemplary embodiment ofthe present invention may be implemented to various shapes, and thiswill be described in detail through the various exemplary embodimentsthat will be described below.

First Exemplary Embodiment

FIG. 6 is a block diagram illustrating a voltage measurement device forthe fuel cell state diagnosis according to a first exemplary embodimentof the present invention. Referring to FIG. 6, it may be assumed thatthe fuel cell state diagnosis frequency for the fuel cell statediagnosis according to the first exemplary embodiment of the presentinvention is about 1 kHz (e.g., a few hundreds to a few thousands Hz).

The signal measurement unit 120 may include a high pass filter (HPF)having a cut-off frequency of 980 Hz and a voltage measurement circuit,and may be configured to measure a first AC voltage to determine andmeasure the fuel cell state diagnosis signal. The noise measurement unit130 may include a band pass filter (BPF) having a pass band of about800˜1200 Hz and a voltage measurement circuit, and may be configured tomeasure a second AC voltage to determine and measure the fuel cell statediagnosis noise. Since the configuration details of each of the elementsin the fuel cell state diagnosis method using the voltage measurementdevice according to an exemplary embodiment of the present inventionthat will be described below may be integrated into a single fuel cellstate diagnosis system 100, the fuel cell state diagnosis system 100will be described mainly.

FIG. 7 is a flowchart illustrating a fuel cell state diagnosis methodusing a voltage measurement device according to the first exemplaryembodiment of the present invention. Referring to FIG. 7, when the fuelcell state diagnosis is started (step, S101), the fuel cell statediagnosis system 100 may be configured to measure a first AC voltage fordiagnosing the fuel cell state diagnosis signal of 1 kHz by collectingthe fuel cell voltage signal of about 980 Hz or more using the signalmeasurement unit 120 (step, S102). The amplitude of 1 kHz of the signalmeasured in the signal measurement unit 120 may be defined as a signalregion A_(signal) (1 kHz).

In particular, since the cut-off frequency of about 980 Hz of the signalmeasurement unit 120 is about the same as the state diagnosis frequency1 kHz in the frequency domain, the hum noise caused by the noise near tothe state diagnosis frequency 1 kHz may be efficiently removed. The fuelcell state diagnosis system 100 may be configured to measure a second ACvoltage to determine and measure the fuel cell state diagnosis noise ofabout 800˜1200 Hz by collecting the fuel cell voltage signal of about800˜1200 Hz region using the noise measurement unit 130 (step, S103).The overall amplitude of 800˜1200 Hz of the signal measured in the noisemeasurement unit 130 may be defined as a noise region A_(noise) (800Hz˜1200 Hz, excluding 1000 Hz).

The fuel cell state diagnosis system 100 may be configured to calculatethe SNR according to the signal region of the first AC voltage and thenoise region of the second AC voltage that are measured above (step,S104). The SNR according to the signal region of the first AC voltageand the noise region of the second AC voltage may be calculated usingEquation 2 above

$\left( {{SNR}\overset{.}{=}\frac{A_{signal}\left( {1\mspace{14mu}{kHz}} \right)}{A_{noise}\left( {{800\mspace{14mu}{Hz}} \sim {1200\mspace{14mu}{Hz}}} \right)}} \right),$and the SNR value has values from 0 to 1.

The fuel cell state diagnosis system 100 may be configured to comparethe calculated SNR value with a predetermined reference value (e.g.,about 0.7). When the SNR value is greater than the reference value (Yesin step, S105), the fuel cell state diagnosis system 100 may beconfigured to determine the acquired fuel cell state diagnosis data tobe reliable, and apply the data to the control of a fuel cell vehicle(e.g., to drive the vehicle) (step, S106). When the SNR value is lessthan the reference value (No in step, S105), the fuel cell statediagnosis system 100 may be configured to determine the correspondingfuel cell state diagnosis data to be unreliable, and may be configuredto discard the data (step, S107). The fuel cell state diagnosis system100 may repeat the procedure until the fuel cell state diagnosis isterminated by return.

Second Exemplary Embodiment

Meanwhile, FIG. 8 is a block diagram illustrating a voltage measurementdevice for the fuel cell state diagnosis according to a second exemplaryembodiment of the present invention. Referring to FIG. 8, it may beassumed that the fuel cell state diagnosis frequency for the fuel cellstate diagnosis according to the second exemplary embodiment of thepresent invention is about 1 kHz (e.g., a few hundreds to a fewthousands Hz).

The signal measurement unit 120 may include a HPF having a cut-offfrequency of about 980 Hz and a voltage measurement circuit, and may beconfigured to measure a first AC voltage to determine and measure thefuel cell state diagnosis signal. The noise measurement unit 130 may beconfigured to measure the high voltage of the fuel cell stack 110 thatis not directly measurable through a voltage dividing circuit in which aplurality of resistors are arranged in parallel, and may be configuredto measure a second AC voltage to determine and measure the fuel cellstate diagnosis signal. The noise measurement unit 130 is the voltagedividing circuit for measuring both of DC voltage and AC voltage, andmay be configured to measure the high voltage of about 450 V bydecreasing the voltage to about 5 V˜10 V. In addition, the noisemeasurement unit 130 may be configured to measure the voltage deviationV_(fc) that is the difference between the maximum voltage value and theminimum voltage value among the second AC voltage values measuredthrough the voltage dividing circuit.

FIG. 9 is a flowchart illustrating a fuel cell state diagnosis methodusing a voltage measurement device according to the first exemplaryembodiment of the present invention. Referring to FIG. 9, when the fuelcell state diagnosis is started (step, S201), the fuel cell statediagnosis system 100 may be configured to measure a first AC voltage fordiagnosing the fuel cell state diagnosis signal of 1 kHz by collectingthe fuel cell voltage signal of about 980 Hz or more using the signalmeasurement unit 120 (step, S202). The amplitude of 1 kHz of the signalmeasured in the signal measurement unit 120 may be defined as a signalregion A_(signal) (1 kHz).

In particular, since the cut-off frequency of about 980 Hz of the signalmeasurement unit 120 is about the same as the state diagnosis frequency1 kHz in the frequency domain, the hum noise caused by the noise near tothe state diagnosis frequency 1 kHz may be efficiently removed. The fuelcell state diagnosis system 100 may be configured to measure voltage ofthe fuel cell using the noise measurement unit 130 and perform aspectrum analysis, and measure a second AC voltage to determine andmeasure the fuel cell state diagnosis noise of a predetermined noiseregion (e.g., of about 800˜1200 Hz) (step, S203). The overall amplitudein the predetermined noise region may be defined as a noise regionA_(noise) (800 Hz˜1200 Hz, excluding 1000 Hz).

In particular, the noise measurement unit 130 may be configured tomeasure the voltage deviation V_(fc) that is the difference between themaximum voltage value and the minimum voltage value among measuredvoltages of fuel cell. The fuel cell state diagnosis system 100 may beconfigured to calculate the SNR according to the signal region of thefirst AC voltage and the noise region of the second AC voltage that aremeasured above (step, S204). The SNR according to the signal region ofthe first AC voltage and the noise region of the second AC voltage maybe calculated using Equation 2 above

$\left( {{SNR}\overset{.}{=}\frac{A_{signal}\left( {1\mspace{14mu}{kHz}} \right)}{A_{noise}\left( {{800\mspace{14mu}{Hz}} \sim {1200\mspace{14mu}{Hz}}} \right)}} \right),$and the SNR value has values from 0 to 1.

When the calculated SNR value is greater than a predetermined referencevalue (e.g., about 0.7) (Yes in step, S205) and the voltage deviationV_(fc) is less than a predetermined reference deviation (e.g., about 50V) (Yes in step, S206), the fuel cell state diagnosis system 100 may beconfigured to determine the acquired fuel cell state diagnosis data tobe reliable (step, S207). In other words, the fuel cell state diagnosissystem 100 may be configured to determine the fuel cell state diagnosisdata to be reliable when both of the condition that the signal for fuelcell diagnosis and the SNR value is greater than the predeterminedreference value and the condition that the voltage deviation V_(fc) isless than the predetermined reference deviation are satisfied.

Furthermore, when the SNR value is less than the reference value (No instep, S205) or the voltage deviation V_(fc) exceeds the predeterminedreference deviation (No in step, S206), the fuel cell state diagnosissystem 100 may be configured to determine the corresponding fuel cellstate diagnosis data to be unreliable, and may be configured to discardthe data (step, S208). The fuel cell state diagnosis system 100 mayrepeat the procedure until the fuel cell state diagnosis is terminatedby return.

Third Exemplary Embodiment

FIG. 10 is a block diagram illustrating a voltage measurement device forthe fuel cell state diagnosis according to a third exemplary embodimentof the present invention. Referring to FIG. 10, the third exemplaryembodiment of the present invention is similar to the first exemplaryembodiment, but different in that the signal measurement and the noisemeasurement are performed using a single integrated measurement unit150. In the description below, it may be assumed that the fuel cellstate diagnosis frequency for the fuel cell state diagnosis is 1 kHz(e.g., a few hundreds to a few thousands Hz). The integrated measurementunit 150 may include a BPF having a pass band of about 800˜1200 Hz and avoltage measurement circuit, and may be configured to measure a first ACvoltage to determine and measure the fuel cell state diagnosis signaland a second AC voltage to determine and measure the fuel cell statediagnosis noise.

Meanwhile, FIG. 11 is a flowchart illustrating a fuel cell statediagnosis method using a voltage measurement device according to thethird exemplary embodiment of the present invention. Referring to FIG.11, when the fuel cell state diagnosis is started (step, S301), the fuelcell state diagnosis system 100 may be configured to measure a first ACvoltage to diagnose the fuel cell state diagnosis signal of 1 kHz bycollecting the fuel cell voltage signal using the integrated measurementunit 150 (step, S302). The amplitude of 1 kHz of the signal measured inthe integrated measurement unit 150 may be defined as a signal regionA_(signal) (1 kHz).

The fuel cell state diagnosis system 100 may be configured to measure asecond AC voltage to diagnose the fuel cell state diagnosis noise in thefuel cell voltage signal of about 800˜1200 Hz region using theintegrated measurement unit 150 (step, S303). In particular, the overallamplitude of about 800˜1200 Hz of the signal measured in the integratedmeasurement unit 150 may be defined as a noise region A_(noise) (800Hz˜1200 Hz, excluding 1000 Hz). The fuel cell state diagnosis system 100may be configured to calculate the SNR according to the signal region ofthe first AC voltage and the noise region of the second AC voltage thatare measured above (step, S304). The SNR according to the signal regionof the first AC voltage and the noise region of the second AC voltagemay be calculated using Equation 2 above

$\left( {{SNR}\overset{.}{=}\frac{A_{signal}\left( {1\mspace{14mu}{kHz}} \right)}{A_{noise}\left( {{800\mspace{14mu}{Hz}} \sim {1200\mspace{14mu}{Hz}}} \right)}} \right),$and the SNR value has values from 0 to 1.

The fuel cell state diagnosis system 100 may then be configured tocompare the calculated SNR value with a predetermined reference value(e.g., about 0.7). When the SNR value is greater than the referencevalue (Yes in step, S305), the fuel cell state diagnosis system 100 maybe configured to determine the acquired fuel cell state diagnosis datato be reliable and acquire the data, and apply the data to the controlof a fuel cell vehicle (step, S306). When the SNR value is less than thereference value (No in step, S305), the fuel cell state diagnosis system100 may be configured to determine the corresponding fuel cell statediagnosis data to be unreliable, and discard the data (step, S307). Thefuel cell state diagnosis system 100 may repeat the procedure until thefuel cell state diagnosis is terminated by return.

Various exemplary embodiments of the present invention are described sofar, but the present invention is not limited to the exemplaryembodiments described above, the substitution or the modificationbetween the exemplary embodiments is also available. For example, in thesecond exemplary embodiment, is the system may be determined to bereliable when the state diagnosis data satisfies the condition that thevoltage deviation V_(fc) is less than the predetermined referencedeviation, and this determination may also be applied to other exemplaryembodiments. Accordingly, there is a merit of removing the noise thatmakes the normal diagnosis impossible in the interval where the fuelcell voltage is rapidly changed.

Meanwhile, FIGS. 12A-12B illustrate an operational effect of a voltagemeasurement device according to an exemplary embodiment of the presentinvention. Referring to FIG. 12A, in the graph when the presentinvention is not applied, there are very drastic measurement deviationin the impedance measurement owing to the problem of operatingfrequencies mixture of fuel cell vehicle electric compartments(operating frequency of fuel cell state diagnosis apparatus—about 500Hz, operating frequency of fuel cell vehicle electric compartments—about400˜600 Hz) and the problem that the state diagnosis apparatus frequencyis buried when the fuel cell voltage is rapidly changed.

Further, referring to FIG. 12B, in the graph when the exemplaryembodiments of the present invention are applied, the effect of removingthe impedance measurement deviation may be identified by acquiring onlythe state diagnosis data of which signal to noise ratio is greater thana predetermined value. Accordingly, the reliability of fuel cell statediagnosis may be improved by calculating the SNR value of the fuel cellstate diagnosis data which are calculated using the respectivemeasurement voltages of the signal measurement unit and the noisemeasurement unit and applying the state diagnosis data to the vehiclediagnosis and control when the SNR value is greater than a predeterminedvalue. In addition, the noise affecting the normal diagnosis may beremoved by determining the state diagnosis data that satisfies thecondition that the voltage deviation of the noise is less than thepredetermined reference deviation to be reliable.

The exemplary embodiments of the present invention are not onlyimplemented through the apparatus and/or method described above, but maybe implemented through a program for realizing the function thatcorresponds to the elements of the exemplary embodiments of the presentinvention, a medium in which the program is stored, and so on. And suchan implementation may be easily done by a skilled person in the art fromthe description of the exemplary embodiments. While this invention hasbeen described in connection with what is presently considered to beexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments, but, on the contrary, isintended to cover various modifications and equivalent arrangementsincluded within the spirit and scope of the appended claims.

What is claimed is:
 1. A system for diagnosing a state of fuel cell,comprising: a signal measurement unit having a high pass filter with apredetermined cut-off frequency and a voltage measurement circuit, andconfigured to measure a first alternating current (AC) voltage tomeasure the fuel cell state diagnosis signal; a noise measurement unithaving a band pass filter with a predetermined pass band and a voltagemeasurement circuit, and configured to measure a second AC voltage tomeasure the fuel cell state diagnosis noise; and a controller configuredto calculate a signal to noise ratio (SNR) of fuel cell state diagnosisdata based on the first AC voltage and the second AC voltage, determinethe corresponding fuel cell state diagnosis data to be reliable when theSNR value is greater than a predetermined reference value, and apply thefuel cell state diagnosis data to a control of a fuel cell vehicle,wherein the noise measurement unit defines the band neighboring thesignal frequency except the fuel cell state diagnosis signal frequencyto be a noise region.
 2. The system for diagnosing a state of fuel cellof claim 1, wherein the controller is configured to discard the fuelcell state diagnosis data when the SNR value is less than the referencevalue.
 3. The system for diagnosing a state of fuel cell of claim 1,wherein the controller is configured to calculate the SNR by dividingthe signal region of the first AC voltage by the noise region of thesecond AC voltage.
 4. The system for diagnosing a state of fuel cell ofclaim 1, wherein the signal measurement unit has the cut-off frequencyof about 980 Hz that is about the same as the state diagnosis frequency1 kHz, whereby a hum noise caused by the noise near to the fuel cellstate diagnosis frequency is removed.